The electron gun of a cathode ray tube (CRT) consists of two sections--the triode and the lens. The lens focuses the emitted electrons using three typically cylindrical elements aligned end to end on the gun axis with the triode. The triode forms the electron emitting section of the gun and comprises an electron source cathode, control grid and accelerator grid.
In operation, the cathode is heated to boil off free electrons. The control grid of the gun alternately enables and disables the electron flow as a function of negative potential. Various combinations of bias control are effective for operating the triode. For example, the electron flow from the cathode can be controlled by means of the video signal while holding the control grid at a constant cut-off potential relative to the accelerator grid. Alternatively, the video signal can be applied to the control grid while the cathode potential is maintained constant, or the video signal can be applied to the control grid and cathode with opposite phase.
Optimum performance of projection CRT guns has been found to occur with the cut-off (i.e. no electron flow) potential between the cathode and control grid to a potential maintained at a predetermined value (e.g. typically 150 volts). This is done by biasing the accelerator grid to a potential (e.g. 400 volts to 1000 volts) at which electron flow is barely stopped when the cathode and control grid potential is set at the aforementioned cut-off potential.
Thus, when the potential between the control grid and cathode is decreased from the cut-off value, electrons are caused to flow from the cathode to the phosphor coating on the viewing surface of the CRT, resulting in generation of a visual image. The maximum allowable current flow occurs when the potential between the control grid and cathode is zero volts.
According to the prior art, wide dynamic range video amplifiers have been utilized to control the potential between the cathode and control grid of a CRT electron gun. One prior art approach utilizes cascode video amplifiers incorporating high voltage transistors. However, it has been found that such high voltage video transistors are characterized by limited frequency response and therefore incapable of providing the wide dynamic range and high bandwidth required for high resolution projections systems.
According to another prior art approach, a pair of smaller cascode amplifiers have been utilized for driving the cathode and control grid with opposite phase signals. However, this approach results in phase problems due to the delay variations between response times of the two cascode amplifiers.